As a method for quantitatively determining a reducing substance in a test specimen or a reducing substance formed by an enzyme reaction, etc. from a biological component in a biological specimen, methods wherein the reducing substance is reacted with iron (III) ions to reduce the iron (III) ions, and the formed iron (II) ions or residual iron (III) ions are detected/quantitatively determined with a metal indicator specific to these iron ions, have been known.
For example, a method wherein hydrogen sulfide or sulfide ions in a test specimen can be quantitatively determined conveniently with high sensitivity is disclosed in JP-A-2000-338096.
However, it was found that in the conventional methods, when a biological specimen such as serum or plasma and an environmental specimen such as sewage water or river water are the subject of measurement, accurate quantitative determination of the reducing substance might sometimes be difficult due to the influences of other components coexisting in the test specimen.